The present invention relates to isolator devices and more specifically to micro electromechanical (MEM) isolator devices providing isolation between at least one input signal and an output signal.
Isolator devices (isolators) are often used to provide electrical or acoustical separation between two locations or environments. Isolators are commonly found in factory and telecommunication applications to prevent interaction between circuits on either side of the isolator.
Prior art isolators include optical coupling devices that convert an electrical signal to an optical signal and then convert the optical signal back to an electrical output signal. In optical coupling systems, the electrical signal drives a light emitting diode to generate light pulses that are received at a detector. The detector then converts the light pulses back into an electrical signal. Since the detector output is electrically isolated from the diode, transients are not propagated from the input to the output. Unfortunately, optical isolators are expensive to manufacture and operate. Optical isolators use expensive gallium arsenide (GaAs) substrates and exhibit high operating power and limited lifetime. Further, optical isolators are not well suited to analog applications since it is difficult to transfer signal analog voltage signals due to the difficulty in detecting variations in the light pulse responsive to intermediate voltage levels.
Isolation transformers are well known and used in many applications since they have low power consumption and provide magnetic (flux) coupling between one or more pairs of circuits without introducing either significant ohmic (conductive) or electrostatic (capacitive) coupling. The isolation transformer has low power requirements, but transformers are heavy, bulky discrete components that depend on separate wire windings on a steel core. Although the windings are electrically insulated from one another, a transformer may transfer high frequency transients from one environment or circuit to the next so the protected environment may require additional protection circuits to filter out the transients. Further, although transformers are able to convert alternating current (AC) to direct current (DC), transformers are unable to provide isolation for DC to DC applications.
Capacitive coupling devices also have low power consumption but similarly have large discrete electrical components that add to system size and weight and are susceptible to electrical failure if stressed under certain conditions. Further, capacitive coupling devices inherently couple harmful transients from the input to the output.
Further, with many prior art isolators, an additional external circuit is required to debounce the output signal to prevent introduction of spurious noise. This additional circuit increases power consumption and cost to isolate the desired signal.
Notwithstanding the problems with the various types of isolators known in the art, isolators are required in a variety of applications. For example, in tele-communication applications, it is common for signals transmitted through a switching network to have voltage and current levels that are incompatible with telephone or computer devices such as modems and similar equipment. Accordingly, an isolator may be used to isolate the head end of the network from remote terminal equipment while still permitting the signal information content to be transmitted between different electrical environments. Further, since environmental factors can affect the signal in transit, there is a need to remove potentially harmful transient signals (such as lightning induced transients) before the terminal equipment is damaged. Further still, it is also desirable to separate circuit grounds so that improperly grounded equipment will not disrupt the operation of the remainder of the switching network.
Similar applications arise in the industrial control environment since automation is becoming an increasingly important part of producing high quality products at competitive prices. As processing equipment becomes more sophisticated and interconnected to other equipment by local area networks or other communication means, control and processing information is generated that must be timely transferred to a server or other processing equipment as well as a noise-free ground reference. In general, isolators are often employed where a signal is generated in a first hostile environment and it is necessary to convey the information content of the signals to a second environment without conveying those signal parameters that are potentially damaging. As noted above, another main function of isolators is to isolate grounds between electrical circuits that are in communications with each other. Having separate matched zero potential conductors on one side compared to the noisy ground from the input other is desirable for noise and safety reasons.
Thus, whatever the advantages of the prior art isolator devices, there is a need for an inexpensive, lightweight, low power, isolator device that has isolation characteristics comparable to prior art isolator devices.
The present invention relates to an isolator based on moveable micro electromechanical (MEM) structures. The general principle is to have at least one electrical input signal control an output signal by way of mechanical motion of an electrically insulating MEM structure. The MEM isolator device comprises a moveable platform suspended above a substrate and a drive and a control capacitor each having a movable electrode supported by the platform and a stationary electrode supported by the substrate. The moveable and stationary electrodes of each capacitor are separated by an air gap so the capacitance changes as a function of the distance between electrodes of the capacitors. By sensing the change in the output capacitance, it is possible to regenerate the input signal at the output. Electrically insulating coupling between the input and the output signal is achieved by converting electrical energy to mechanical energy. This conversion is achieved by providing an input signal to the drive capacitor to induce electrostatic platform motion. This motion induces a corresponding change in the control capacitance that is detected by a control circuit. The control circuit then produces an output signal that follows the input signal but is electrically isolated therefrom.
More specifically, the drive capacitor is coupled to a signal source and when a signal applied across the drive capacitor, an electrostatic force is generated causing the electrode mounted on the moveable platform to move either toward or away from the fixed electrode. This electrode motion converts electrical energy into mechanical energy. The mechanical motion of the platform transfers the mechanical energy to the control capacitor and since the value of the capacitance is proportional to the distance (1/d) between the electrodes of the capacitor, the capacitance value increases when the gap spacing is decreased. Similarly, as the gap spacing is increased, the capacitance value of the capacitor will decrease. Since the value of the capacitance is proportional to the distance (1/d), the value of the capacitor increases when the gap spacing is decreased and as the gap spacing is increased the capacitance value of the capacitor will decrease. The change in the value of the control capacitor is measurable and used to determine the magnitude of the input voltage. Advantageously, the isolator of the present invention is capable of isolating both AC and DC signals.
In the preferred embodiment, the moveable electrodes of both the drive and the control capacitors are mounted on a common insulating platform so that there is no electrical connection between the drive and control capacitors. The isolator device of the present invention provides isolation voltage levels on the order of several thousands of volts limited only by the breakdown voltage of the insulating material. The isolator of the present invention protects one side of a circuit from harmful signal components while permitting the transfer of the signal content. Further, separate grounds are readily provided with the present invention since the isolator may couple either digital or analog signals from one environment to another.
Depending on design parameters, a data rate of ten thousand bits per second (10,000 bps) is readily achieved. However, if significantly higher data rates are necessary, the signal size will degrade since the resonant frequency of the structure is inversely proportional to square root of the mass. Thus, higher signal rates require smaller capacitors resulting in smaller signal size. Compared to optical isolator devices, active isolators or transformers, power consumption is significantly reduced with the present invention with minimal power consumption on the order of only a few microwatts. The isolator device of the present invention may be integrated together with control circuits as a single component using flip-chip technology. Advantageously, the present invention also provides a signal debounce feature that filters spurious high frequency signals.
The MEM structure is fabricated using well-known semiconductor processing techniques to define and release the microstructure from single-crystal silicon material resulting in an isolator device that is small and lightweight.